PRINCIPAL OBJECTIVES: To characterize the major histocompatibility complex (MHC) or B complex in chickens and to relate the MHC to genetic control of disease resistance. We have discovered a genetic marker (GAT) for the immune response region (Ir) of the B complex in chickens. GAT is the amino acid polymer, Glu-Arg-Tyr and when used as an antigen, both high and low responders are found. We have produced 4 haplotypes, GAT-low and GAT-high in both B1B1 and B19B19 blood groups. Also we find that GAT-low for either blood group has higher general mortality, and suffers higher death loss when challenged with either Marek's disease virus or Rous Sarcoma virus. Our plan is to compare these haplotypes for serum levels of IgG, IgM and IgA and for rate of formation of Ig in response to antigenic stimulation and to study the role of Ia alloantigens in immune response mechanisms. Reagents will be produced by injecting Bursa of Fabricious B cells from GAT-low and GAT-high donors into selected recipients. Identification of anti-alloantisera will be by indirect immune fluorescent staining. Other work planned includes: compare haplotypes as to their effectiveness in lymphocyte stimulation using different mitogens. The question is whether GAT-low responders suffer a defect in either B or T cells. Finally, we plan to use the mouse hybridoma technic to develop monoclonal blood typing reagents. The aim is to search for a dual histocompatibility system in fowl corresponding to H-2K and H-2D in the mouse.